Simultaneous digitizing apparatus for an acoustic tool

ABSTRACT

For use in an acoustic logging tool, an apparatus which digitizes simultaneously obtained acoustic signals is set forth in the preferred and illustrated embodiment. The device cooperates with N acoustic receivers in a sonde. After an acoustic pulse is transmitted, data is observed at all N acoustic receivers. This apparatus comprises a multiplexer which is connected to the several receivers. The several input signals are multiplexed, thereafter input to a digital data converter forming a procession of output digital words, and the words are stored in a selected order in a digital data buffer. They are delivered to the surface through a telemetry transmitter at a slower rate than the rate at which the data is created. In addition, a transmitter monitor is included. This provides a signal alternately digitized for a specified interval to enable coordination of the data reduction from the acoustic receivers in contrast with the timing of the transmitted acoustic pulse.

BACKGROUND OF THE DISCLOSURE

Acoustic logging tools utilize an acoustically coupled piezoelectrictransducer to convert acoustic waves into output electrical signals. Atypical acoustic logging tool incorporates an acoustic transmitter whichpropagates a pulse into the adjacent formations. A return pulse isreceived at the device. An input pulse is observed at each acousticreceiver. Better data interpretation can be obtained if there aremultiple acoustic receivers. In a typical device there are N acousticreceivers, and they output data collectively indicating more subtlerelationships. One subtle relationship is in the relative phase betweenthe various receivers. The delay time of the propagated wave front inarriving at the different receivers is also important. A multitude ofdata is made available by such a device.

The data burst occurs in about 500 to 1,500 microseconds after the pulseis transmitted. Simultaneous data reception and telemetry is especiallydifficult over a monocable. A monocable is a cable for supporting adownhole logging tool in a sonde wherein two conductors are included inthe cable. While one functions as ground, power is delivered on theother conductor to be transmitted from the surface to the sonde.Additionally, this pair of conductors is used to transmit data from thetool to the surface. This two conductor system provides a somewhatlimited band width for data transmission, and it is not possible tocrowd full data through the monocable in real time from N acousticreceivers.

One data transfer procedure has accommodated this limited data bandwidth in the past is use of multiple firings of the transmitter, eachfiring dedicated to a single acoustic receiver. Thus, four separatelycreated acoustic pulses would be propagated into the adjacentformations, and each of the four acoustic receivers would besequentially operated to provide an output signal. This typically occursas the tool is in transit up the borehole. There is an inevitable shiftin position of the sonde between pulses. This movement makes it somewhatdifficult to implement various data reduction procedures using Nacoustic receivers where there has been a shift in the position of theacoustic transmitter and the respective receivers because each isworking with different transmitted pulses. This has created difficultiesin data reduction; the data reduction requires shifting to obtain timecoincidence of the transmitted pulses. Since the shifted data is notwhat really happened, such data reduction techniques add to thecomplexity of interpretation. One important data reduction technique iscross correlation of the transmitted pulse and the one received pulse.The present invention enhances cross correlation.

The present apparatus and method enable use of multiple simultaneouslyoperative acoustic receivers supported in acoustic well logging tools.The tool is lowered in a borehole. At a desired depth, the acoustictransmitter is operated to form a pulse. The pulse is transmitted intothe adjacent formations and various acoustic signals are observed at Nacoustic receivers on the tool. The various received signals are timemultiplexed and stored in memory after conversion into a set of digitalwords. For each of the N acoustic receivers, the received signal can bereconstructed at the time of data reduction and interpretation.Moreover, the data is stored so that it can be subsequently transferredto the surface on a monocable by means of a suitable telemetrytransmitter. With the data from N acoustic receiver signals in memory,the data can be removed from memory at a rate which permits it to betransmitted in the narrow width band available in a monocable. As willbe understood, broad band transmission to enable high speed transmissionof N simultaneously received acoustic signals is obtained only at theprice of a more expensive apparatus capable of broad band transmission.The present apparatus enables the time log between acoustic events to beused for data transmission of N acoustic received events.

Many additional objects and advantages of the present apparatus will bemore readily apparent on consideration of the device after its detaileddescription below. Such device and a method of obtaining data are setforth in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features, advantages andobjects of the present invention are attained and can be understood indetail, more particular description of the invention, briefly summarizedabove, may be had by reference to the embodiments thereof which areillustrated in the appended drawings.

It is to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 shows the acoustic logging system constructed in accordance withthis disclosure particularly featuring a multiplexed input to a digitaldata converter for temporarily buffering digitized data for subsequenttransfer to the surface; and

FIG. 2 is a timing chart showing the timed sequence of various signalsin a system having four acoustic receivers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Attention is first directed to FIG. 1 of the drawings where an acousticlogging device is indicated generally by the numeral 10. It is supportedon a monocable in a borehole 14 to thereby obtain acoustic logging data.The monocable extends upwardly to the surface to pass over a sheave 16.The sheave 16 directs the cable to a reel 18 which spools and storesseveral thousand feet of the cable. The cable is connected to a receiver20. The receiver receives the acoustic logging data and transfers it toa recorder 22. The recorder 22 stores the logging data in a suitablemedium such as on magnetic tape. The recorder 22 is connected by amechanical or electronic means 24 to the sheave for the purpose ofdetermining the depth of the sonde 10 in the borehole 14. This enablesthe data to be correlated to the depth in the well. In typicaloperations, the sonde 10 is lowered to the bottom of the well and thenis retreived from the well by spooling the cable onto the reel 18. Asthe sonde is raised in the well, acoustic pulses are transmitted in acontrolled sequence. Data is obtained and recorded by the recorder 22 asa function of depth of the sonde 10 in the borehole.

The acoustic logging device incorporates an acoustic transmitter whichis typically a piezoelectric crystal. The transmitter is identified bythe numeral 26. The transmitter is immediately adjacent to an acousticreceiver 28. It functions as a transmitter monitor. It obtains a signaldependent on transmission; the acoustic receiver 28 provides an outputsignal indicative of transmission. It will be used in a fashion to bedescribed. The acoustic logging tool 10 additionally supports severalpiezoelectric receivers. Moving from the bottom to the top of the tool,the numeral 30 identifies one receiver. A similar receiver isincorporated at 32. Additional receivers are shown at 34 and 36. Theyare preferably identical in operation and construction. They differprimarily in their location in the sonde. In particular, they are spacedat known distances from the transmitter.

If a pulse is transmitted into the formation, a received signal isobserved at all four of the receivers. The several signals from theseveral receivers may differ in phase and amplitude. Valuable data isfound in these differences.

The four receivers are input to a multiplexer 38. The multiplexer 38 isprovided with N inputs. In this instance, N is four. As will beunderstood, the number of acoustic receivers can be varied. This numberdepends on the construction of the sonde 10 and the desired data fromits operation. The output signals from the four receivers are analogsignals.

The analog multiplexed signal is input to a next multiplexer 40. Thismultiplexer does not have to be a high speed multiplexer in the samefashion as the multiplexer 38. Preferably, the multiplexer 38 is able tosettle in less than one microsecond so that microsecond scanning speedsfor the N inputs can be accomplished. The multiplexer 40 is providedwith two inputs. One is on the conductor 42 from the acoustic receiver28. That signal is provided for the first designated interval (typicallyup to about two hundred microseconds) after the transmitted pulse hasbeen formed. In other words, transmitter monitoring occurs for the firstfew hundred microseconds of a pulse transmission and reception sequence.During the first microseconds of operation, there is no signal at any ofthe acoustic receivers. It is therefore preferable to record theacoustic transmitter monitor signal to be able to determine timing andshape of the transmitted pulse. Later on, this data will less importantand the more important data furnished from the high speed multiplexer 38is then fed through the multiplexer 40. Multiplexer 40 therefore isinitially operated to transfer only the transmitter pulse signal; thatis ended after an interval, and thereafter the only signals for themultiplexer 40 are from the acoustic receivers. The multiplexer 40 isinput to a high speed analog to digital converter 44. It converts thevariable signals into an output digital signal having a specified wordlength and sign bit. Typically, eight bits in the data words are formed.Greater precision can be obtained at some sacrifice in complexity orspeed.

This apparatus includes a control system 46. The control system 46 timesoperation of all the equipment. The control system is connected to adigital data buffer 48 which is a memory device for storing the data.The data is stored in a rank and file organization to be described. Thedata in the buffer 48 is periodically removed for a telemetrytransmitter 50 to be transferred to the surface through the monocable12.

The control system 46 forms a fire signal for a transmitter firecontroller 52 connected to the transmitter. This enables the correcttiming of the transmitted pulse.

The control system 46 is additionally connected to the multiplexer 38 bymeans of a conductor 54. This gates the multiplexer 38 at a speed to bedetermined by the control system. Likewise, a control signal is providedon the conductor 56. The signal on the conductor 56 is input to themultiplexer switch 40. This instructs the circuit 40 to transfer eitherthe transmitter signal or the multiplexed receiver signals.Additionally, the control system provides a write signal on a conductor58. Data is written through several conductors at 60.

Data is written in an organized fashion in the digital data buffer 48.One organization for the digital data buffer is shown in FIG. 2 of thedrawings. There, the four receivers are labelled on the ordinate. Inaddition, the transmitter monitor is also included. The abscissa ismeasured in microseconds. For the first few microseconds the only signalpotentially available is the transmitter pulse. The transmitter monitor28 provides a signal which is suitably digitized. Thus, the first fewwords into memory come from the transmitter pulse. All of these wordsare serially output from the high speed converter 44 into memory. FIG. 2thus represents the first several words derived from the transmittermonitor and input into memory. It will be observed that the abscissa hasa break to indicate that the number of words so stored can be varied.This is achieved by the control system 46 switching the multiplexer 40.

FIG. 2 thus shows that, after an interval, the last word is obtainedfrom the transmitter monitor 28. The next digital word is from thereceiver 30. In FIG. 2, scanning of the four receivers is shown. In thescale of FIG. 2, a digitized data word is obtained from each of the fourinput signals at a timed spacing of approximately four microseconds perinput. The data rate for the system as a whole is much faster to enablescanning of four acoustic receivers. This data rate thus enables thewords to be interlaced as they are placed in memory in a specifiedsequence. As data words are placed in memory, they are aligned in memoryin the same sequence. Thus, FIG. 2 shows the interlacing of wordssequentially obtained from the several receivers and stored in memory inthe interlaced fashion.

This system should be considered for operation over an interval in whichseveral acoustic pulses are transmitted. Assume as an example tha anacoustic pulse is transmitted, and that the acoustic receiver 28 isoperated for 128 microseconds. The first 128 words into memory are allobtained from the transmitter. The next step (under the control system46) involves multiplexer 38 to multiplex the N inputs. The N inputs areindividually input and digitized as the scanning is repeated. Thepattern of input words for the digital data buffer 48 is preserved inthe buffer.

Assume that the total time frame of collecting data is 1,000microseconds or one millisecond. In that instance, 1,000 data words areobtained. Fortunately, the spacing of the data samples from the fouracoustic channels afford sufficient data points to enable reconstructionof the analog signals at the time of data conversion. In any event, thebuffer 48 stores 1,000 words. Assume further that the acoustic pulseswill occur 500 milliseconds apart. If the logging tool is being raisedin the borehole at a rate of 120 feet per minute or 2 feet per second,then spacing of 500 milliseconds enables data to be safely obtainedapproximately every foot. Recall that the data in the buffer wasobtained over 1,000 microseconds or one millisecond. This data can thenbe transferred over a time interval less than about 490 milliseconds, amarkedly slower rate of transfer. This slow rate enables the telemetrytransmitter to obtain and transmit the data (1,000 data words) from thebuffer to the surface. This data transfer occurs at such a slow ratethat the pass band of the monocable imposes no particular limitation onthe operation of the system. In fact, this slow rate of transfer of datais advantageous because it then enables the use of a monocable. Themonocable is able to transmit this data through the telemetrytransmitter in the sonde to the receiver at the surface without datadegradation. The clarity and quality of data transfer is thus enhancedby the slow speed of transfer. While high speed transfer potentiallycould be had, the slow transfer is desirable to enable the use of themonocable.

Scale factors have been used above. Needless to say, they can be varied.For instance, the time duration of data capture can be shortened orlengthened. The sampling rate can also be shortened or lengthened. Thenumber of data words can be varied so long as it does not exceed thecapacity of the buffer 48. A typical buffer might hold perhaps 4K datawords typically having an eight bit length. If needed, the memory canhold 16K or 32K data words. This is an adjustable factor which can bechanged merely by placing a larger memory in the device. All of the datafrom logging thousands of feet of borehole, however, cannot simply bestored in memory. Rather, this apparatus enables the data to be storedmomentarily in memory and transmitted by the transmitter 50 to thesurface on the monocable at a time rate which is acceptable in light ofthe band pass of the monocable and the velocity of movement of the sonde10 up the borehole.

While the foregoing sets forth the method and apparatus of the presentinvention, the scope is determined by the claims which follow.

What is claimed is:
 1. For use in a well borehole to obtain multiplesimultaneous data signals from N acoustic receivers carried by anelongated acoustic logging sonde sized and adapted for passage in thewell borehole and suspended by a well logging cable having a singleacoustic data conductor, the multiple simultaneous signals formed inresponse to a single acoustic energy pulse from an acoustic transmittercarried by the sonde, a data collection system comprising:(a) N acousticreceivers supported by an acoustic logging sonde forming N timeoverlapping acoustic logging output signals from the receivers where Nis an integer; (b) first multiplexing means having N inputs connected tothe outputs of said N acoustic receivers; (c) second multiplexing meansconnected to said first multiplexing means; (d) digital converter meansoperating at a first rate and connected to said second multiplexingmeans for forming a time based series of digital words representative ofthe output signals of said N receivers; (e) digital data buffer means insaid sonde and connected to said digital converter means for recordingin a formatted order a series of digital words from said convertermeans; (f) an acoustic transmitter monitor means for monitoring acoustictransmitter pulses transmitted by the acoustic transmitter and supplyingthe monitored transmitter pulses to said second multiplexing means; (g)control means connected to said first and second multiplexing means forcontrolling operation of said first and second multiplexing means toform a series of digital words representative of; (1) the monitoredtransmitter pulses from said acoustic transmitter monitor, and (2) the Nacoustic logging output signals; (h) timing means incorporated with saidcontrol means to time digitizing by said digital converter means tooperate both said first and second multiplexing means to form: (1) afirst series of digital words representing the monitored transmitterpulses; and (2) a second and later series of digital words representingthe N acoustic logging output signals; and (i) telemetry transmittingmeans for telemetering the first and second series of digital words fromsaid buffer means along the single acoustic data conductor in the cableat a second rate which rate is less than the first rate such that asingle acoustic energy pulse from said acoustic transmitter generates Ndigital representations of the acoustic energy received at said Nacoustic receivers corresponding to the arrival at each receiver of asingle acoustic energy pulse.
 2. The apparatus of claim 1 wherein saidacoustic monitor is located in the sonde connected to an acoustictransmitter to detect and form a signal representative of an acousticpulse formed by the acoustic transmitter and transmitted by saidacoustic transmitter, said monitor forming a signal subsequently inputto second multiplexing means to be converted into digital words by saidconverter means.
 3. The apparatus of claim 1 wherein the N acousticreceivers are longitudinally spaced along the sonde from one another. 4.The apparatus of claim 3 wherein the acoustic transmitter is locatednear one end of the sonde.
 5. The apparatus of claim 1 including meansfor directing digital words from said digital converter means intospecified addresses in said digital data buffer means.
 6. A method ofacoustic logging in a well borehole below a surface with an acousticpulse transmitter on a sonde supported by a cable wherein the cable hasa single acoustic data signal conductor and N multiple acousticreceivers on the sonde to observe overlapping received acoustic signals,the method comprising the steps of:(a) transmitting an acoustic pulse bya transmitter; (b) forming a signal by monitoring the acoustic pulsetransmitter during operation thereof ; (c) multiplexing and digitizingthe formed signal for a first time interval beginning at time equalszero to form a series of digital words representing the acoustic pulse;(d) storing the digital words in a memory in the sonde; (e) listeningfor overlapping acoustic signals resulting from the transmitted acousticpulse at N acoustic receivers along the well borehole and forming ananalog signal from each of the N receivers; (f) multiplexing theoverlapping N received analog signals at first and second rates; (g)digitizing the multiplexed N analog signals into a series of digitalwords encoding the N analog signals; (h) storing the digital wordsencoding the N analog signals in a specified sequence in the memory; and(i) telemetering from the sonde along the single acoustic data signalconductor along the cable supporting the sonde to the surface digitalwords in the memory representing the monitored acoustic transmitterpulse and the N received signals, wherein the telemetering rate isslower than the first rate.
 7. The method of claim 6 wherein themonitored signal from the acoustic transmitter is converted into digitalwords for a first time interval from time equals zero and thereafterdigitizing only the N acoustic receiver signals for a second timeinterval.
 8. The method of claim 6 wherein digital words after the firstinterval are digital words from only the N acoustic receivers.
 9. Themethod of claim 6 wherein the digitizing forms the digital wordsoccurring during the first interval indicative of only the monitoredacoustic transmitter signal .
 10. The method of claim 9 wherein digitalwords during the second interval are representative of only the receivedacoustic analog signal.
 11. The method of claim 6 wherein N is four; andthe monitored acoustic transmitter signal is defined in the digitalwords telemetered from memory in the sonde to enable cross correlationof the data from the N acoustic receivers.
 12. The method of claim 11wherein the step of multiplexing includes sequential scanning of Nacoustic analog signals at a multiplexing scanning rate.